Power Allocation and Subcarrier Pairing for OFDM-Based AF Cooperative Diversity Systems

In this paper, a two-hop amplify-and-forward cooperative diversity system using OFDM modulation was considered. We proposed an associated waterfllling power allocation scheme under separate power constraints at source and relay, which allocated source and relay power in an associated fashion. The scheme first allocated power to subcarriers uniformly, then gave an approximate equivalent channel gain model, and last employed the classic waterfilling algorithm to optimize the power allocation at relay and source, respectively. Moreover, we investigated the subcarrier pairing problem and presented a modified sorted subcarrier pairing algorithm, which considered the effect of source-destination link as well as source-relay link. The simulation results show that the associated waterfilling power allocation scheme and the modified sorted subcarrier pairing algorithm both could achieve higher average rate alone, and that the combination of them could improve the performance further.